1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an automobile.
2. Description of the Prior Art
Pellet-shaped or monolithic catalysts are generally used to purify exhaust gas from automobiles. Among harmful components (HC, CO and NO.sub.x) in exhaust gas, purification of HC by means of a catalyst is strongly dependent on the temperature of exhaust gas and needs a high temperature, generally 300.degree. C. or higher, even when a noble metal catalyst is used. Accordingly, HC is difficult to purify by means of a catalyst when the exhaust gas temperature is relatively low, for example, immediately after the engine has been started. Moreover, since a large amount of HC is emitted immediately after starting of the engine, the ratio of HC emitted when the exhaust gas temperature is relatively low (hereinafter referred to as "cold HC") to the whole emission is relatively high. Therefore, it has heretofore been a subject to control the emission of cold HC. Although a method wherein the engine is controlled to reduce the amount of HC emitted from the engine has been adopted, this method is unfavorable because it causes lowering in the intrinsic performance of the engine.
To solve the above-described problem, Japanese Utility Model Laid-Open No. 62-5820 (1987) and Japanese Patent Laid-Open No. 57-159908 (1982) disclose one type of exhaust gas purifying apparatus which is designed to adsorb HC in exhaust gas when the exhaust gas temperature is relatively low. Of these two prior arts, Japanese Utility Model No. 62-5820 (1987) uses an adsorbent and a catalyst in such a manner that, when the exhaust gas temperature is relatively low, the adsorbent adsorbs HC, whereas, when the temperature is relatively high, the catalyst purifies both HC desorbed from the adsorbent and HC emitted from the engine.
The above-described conventional exhaust gas purifying apparatus suffers, however, from the following problems. Namely, examples of materials which are proposed in the prior art as being usable adsorbents are .gamma.-alumina, porous glass, activated carbon, silica gel, etc. and it has been impossible to obtain satisfactory adsorption efficiency with these materials. In particular, the adsorption efficiency of these adsorbents lowers considerably when the exhaust gas temperature is high and, therefore, if the exhaust gas temperature is somewhere between a temperature at which the adsorption efficiency starts to lower and a temperature at which purification by means of the catalyst is available, HC will be undesirably emitted without being adsorbed by the adsorbent nor purified by means of the catalyst. For this reason, it has heretofore been impossible to obtain the desired HC purifying efficiency.
Accordingly, it is an object of the present invention to provide an exhaust gas purifying apparatus of such a type that, when the exhaust gas temperature is relatively low, the adsorbent adsorbs HC, whereas, when the temperature is relatively high, the catalyst purifies both HC desorbed from the adsorbent and HC emitted from the engine, wherein the HC adsorption efficiency of the adsorbent in a high-temperature region is improved by a large margin so that the adsorbent is capable of effectively adsorbing HC up to a temperature at which purification by means of the catalyst is available, thereby enabling excellent HC purifying efficiency to be obtained without lowering the engine performance of the automobile.
As a result of exhaustive study of adsorbents having high HC adsorption efficiency, the present inventor has found that zeolites exhibit high HC adsorption efficiency up to relatively high temperature, and further confirmed that among zeolites mordenite and Y-type zeolite have particularly excellent HC adsorption efficiency and are long lasting.